1. Field of the Invention
The present invention relates to a golf club shaft which comprises at least three successive tubular members axially joined together.
2. Description of the Related Art
One type of golf club shaft that has recently been finding widespread use among golf players comprises a tubular body of fiber-reinforced synthetic resin which extends from a butt end as a grip support toward a tip end as a head support. The golf club shaft that is used generally is of a tapered shape whose diameter gradually decreases from the butt end toward the tip end.
The golf club shaft of fiber-reinforced synthetic resin is manufactured by a sheet winding process. According to the sheet winding process, a partly hardened prepreg sheet which is made of fibers impregnated with synthetic resin is wound around a tapered mandrel, and then heat-set to shape. Because the diameter of the shaft varies in the axial direction, the flexional rigidity of the shaft is progressively smaller from the butt end toward the tip end. The rate at which the flexional rigidity of the shaft varies may be changed by increasing or reducing the taper of the shaft, but cannot largely be changed. This imposes a limitation on efforts to place the bending point, i.e., the point of maximum flexibility of the shaft, near the butt end, making the shaft more flexible near the club grip, or near the tip end, making the shaft more flexible near the club head. Therefore, the golf club shaft has no sufficient freedom for design.
Golf clubs that are more flexible near the club grip or the shaft butt end can store energy upon a swing, and hence can produce a powerful ball hit. On the other hand, golf clubs that are more flexible near the club head or the shaft tip end can hit a ball at a large angle and hence along a high trajectory. For this reason, some golf players prefer golf clubs that are more flexible near the club head. However, the conventional golf club shafts made of fiber-reinforced synthetic resin have failed to satisfy such needs of golf players.
Attempts have been made to manufacture a golf club shaft of fiber-reinforced synthetic resin whose local flexional rigidity is largely changed by introducing, between the butt and tip ends thereof, a shaft portion which is thinner or thicker than the butt and tip ends or a shaft portion which has an abrupt increase or reduction in diameter compared with the butt and tip ends.
The sheet winding process cannot be easily applied to make shaft portions of different diameters because it is difficult to wind a prepreg sheet around those shaft portions of different diameters.
According to another process, a prepreg sheet is wound around a tube of synthetic resin, and then the tube is placed in a mold having a mold cavity in the shape of a golf club shaft. Thereafter, air is introduced into the tube to exert an internal pressure thereto while the tube is being heat-set to shape. This process is, however, disadvantageous in that the tube tends to become lower in mechanical strength in regions where it is largely expanded under the internal pressure, and the tube is liable to suffer shape failures.